In nuclear reactor technology, it is customary to enclose the reactor core in a pressure vessel or containment which can be of cylindrical configuration and can be composed of concrete or, more advantageously, can be assembled from a multiplicity of annular elements of cast iron or the like.
Cast iron rings, for example, may be stacked to form the pressure vessel for enclosure and, instead of rings, cast iron segments can be assembled for this purpose.
In either case, the elements are placed under axial prestress by tension elements running through the walls of the vessel in the vertical direction and appropriate covers, bottoms and linings may be applied as required.
To strengthen the pressure vessel against outward forces, it is not uncommon to provide the cylindrical wall of the vessel with inwardly acting prestressing means which can be, for example, cables extending around the circumference of the vessel and tensioned to apply the desired degree of inward prestress. It is also known to apply rings to the periphery of the vessel and to stress the rings inwardly against the vessel wall with one or more turns of highly tensioned cables.
It is known to apply inward-stressing rings of the aforedescribed type with the aid of a fluid-pressure piston/cylinder arrangement which is capable of spreading the ring and enabling the same to be slipped over the vessel. Earlier systems of this type tend to deform the ring into a polygonal configuration which remains when the ring is applied to the vessel. Because of the different frictional characteristics of the various parts of the ring in contact with the vessel, it cannot be assured that a uniform inward prestress will be applied over the entire periphery of the latter. Naturally, prestressing of the vessel in this fashion is unreliable and may pose a safety problem in operation of the reactor or vessel.